Second generation mobile communication indicates transmitting and receiving (transceiving) voice signals in a digital manner, which includes CDMA, GSM, and so on. GPRS advanced from the GSM has been proposed. The GPRS is technology for providing packet switched data service based on the GSM system.
Third generation mobile communication indicates transceiving images and data, as well as voice signals. Third Generation Partnership Project (3GPP) has developed mobile communication system (IMT-2000) technology, and has adopted WCDMA as Radio Access Technology (RAT). In Europe, the IMT-2000 technology and the RAT (e.g., WCDMA) are referred to as Universal Mobile Telecommunication System (UMTS). And, UTRAN is an abbreviated term of UMTS Terrestrial Radio Access Network.
The third generation mobile communication is being evolved into fourth generation mobile communication.
As the fourth generation mobile communication, Long-Term Evolution (LTE) Network technology being standardized in 3GPP, and IEEE 802.16 technology being standardized in IEEE have been proposed. In the LTE, the term of E-UTRAN (Evolved-UTRAN) is used.
The fourth generation mobile communication technology has introduced OFDM (Orthogonal Frequency Division Multiplexing)/OFDMA (Orthogonal Frequency Division Multiple Access). In the OFDM, plural orthogonal subcarriers are used. The OFDM uses an orthogonal characteristic of IFFT(inverse fast Fourier Transform) and FFT(fast Fourier Transform). A transmitter transmits data after performing IFFT, and a receiver restores the original data by performing FFT with respect to a received signal. The transmitter uses IFFT for combination of a plurality of sub carriers, and the receiver uses corresponding FFT for separation of a plurality of sub carriers.
Attempts to increase a cell capacity are ongoing so as to support high capacity service and bi-directional service such as multimedia contents and streaming, in the third or fourth generation mobile communication system.
For increase of the cell capacity, there have been proposed methods for using a high frequency band and reducing a cell radius. If a cell having a small radius (e.g., pico cell) is applied, a frequency band higher than the existing one used in a cellular system can be obtained. This can allow much information to be transmitted. However, in this case, there is a disadvantage that high costs are required since more base stations should be installed on the same area.
As one of the approaches to increase the cell capacity using a small cell, there have been proposed a femto cell.
The femto cell indicates a miniaturized mobile communication base station connected to a mobile communication core network via a broadband network installed inside home, an office, etc. Such femto cell has advantages that indoor coverage is increased, the call quality is enhanced, and various wired/wireless convergence services are efficiently provided.
3GPP WCDMA and LTE groups are performing standardization with respect to the femto cell in the name of Home eNodeB, and 3GPP2 is also actively doing research on the femto cell.
In the 3GPP, the femto cell is categorized into a closed subscriber group (CSG) femto cell, and an open access femto cell according to an accessible status. In case of the CSG femto cell, only terminals (user equipment) allowed to access a corresponding femto cell can access the femto cell. The open access femto cell indicates a femto cell which provides limitless service to all terminals which can receive service.
Also, the 3GPP allocates a CSG ID to a femto cell for identification of a CSG femto cell, and defines Femto-cell White List, the list including CSG IDs of femto cells, the ID by which a corresponding terminal can access USIM (universal subscriber identity module) of a femtocell-supported terminal. A corresponding femto cell periodically transmits the CSG IDs using a control channel, so that a CSG femto cell of a mobile terminal can be identified.
As methods for applying such femto cell to the existing mobile communication network, various structures shown in FIGS. 1 and 2 have been proposed.
FIG. 1 is an exemplary view illustrating a femto-cell based network structure in accordance with the conventional art.
FIG. 1 illustrates a macro base station (M-BS) which provides service onto a wide area, and a plurality of femto base stations (f-BS) installed based on a user.
The femto base station (f-BS) is connected to femtocell network controller (FNC) via the Internet thus to be controlled, and provides service to a user.
The function and specification of the FNC are differently defined in various standards. In this disclosure, the FNC is defined as a cellular network entity having a function to control femto base stations. For instance, in case of 3GPP LTE, the FNC may be implemented as MME(Mobility Management Entity) of a core network, or HeNB Gateway having functions to control femto cells and to perform traffic processing.
A mobile terminal measures signals of neighbor cells, and transmits the measured signals to femto base stations thereof. And, the femto base stations identify and manage the neighbor cells. And, the femto base stations exchange information with each other through a direct link, or through an indirect link via the FNC. And, the femto base station and the macro base station exchange information with each other, via the FNC or RNC (Radio Network Controller), or MME (Mobility Management Entity) which controls the femto base station in a mobile communication network.
Standardization groups such as IEEE 802.16m and 3GPP LTE are doing research on cooperative communication and data transmission between neighbor femto cells.
The first reason why cooperative communication is required between neighbor femto cells is in order to prevent degradation of radio communication environment of each femto cell due to interference, when femto cells are positioned close to each other. Many methods have been proposed in order to solve such interference between the neighbor cells. And, required control data should be transmitted between femto cells.
The second reason why cooperative communication is required between neighbor femto cells is in order to support rapid handover (HO) between two femto cells. In the conventional handover method applied when a terminal moves between macro base stations (Macro BS), handover of the terminal is rapidly performed, because the terminal's data required to perform the handover is transmitted via a backbone network of the macro base stations which can guarantee QoS. However, when handover of the terminal is performed between neighbor femto cells, the femto cells transmit data required to perform the handover, to a neighbor base station, via a cellular core network, through a wired broadband network which cannot guarantee QoS. This may cause the occurrence of handover delay. Research on such inter-femtocell handover is being actively done by 3GPP LTE RAN3 WG, on the topic of “HeNB Mobility Enhancement”.
The third reason why cooperative communication is required between neighbor femto cells is in order to transmit user data between neighbor femto cells. In the occurrence of data communication between terminals connected to neighbor femto cells, respectively, corresponding data is transmitted to a neighbor femto cell, through a broadband network of the femto cell, via a cellular core network, and through the broadband network again. In this case, data transmission delay may occur. In order to solve such problem, required is rapid data transmission via channels directly connected between neighbor femto cells.
Currently, 3GPP LTE SA2 WG is doing research on such problem of direct data transmission between femto cells, on the topic of “Local IP Access (LIPA)”.
The direct communication between femto cells which have been researched so far relates to channel setup using radio resources. However, it is difficult to implement such direct communication due to the following problems, such as a problem of frequency resource allocation, a scheduling problem, a hidden node problem that femto cells are located at positions where direct communication cannot be performed in spite of overlapped coverage.